The present invention relates to a bimetal overload tripping device installed in a circuit breaker, such as an autobreaker.
Taking the autobreaker as an example, a configuration of a circuit breaker in which the present invention is implemented will be described with reference to FIGS. 3(a) and 3(b). In these figures, reference numeral 1 denotes a main body case of a circuit breaker (in the figures, the cover has been removed from the case), 2 is a power supply side main circuit terminal, 3 is a load side circuit terminal, 4 is a handle for opening and closing operations, and 5 is an adjustment dial for adjusting a current flowing through a thermal overload/open-phase tripping device to a rated value described below. The case 1 internally has a breaking section 8 formed of a movable contact shoe 8a, a fixed contact shoe 8b, and an arc extinguish chamber 8c, an opening-and-closing mechanism section 9 for driving the movable contact shoe 8a of the breaking section 8 to an open or a closed position, a thermal overload/open-phase tripping device 10, an electromagnetic instantaneous tripping device 11, and other parts.
The thermal overload/open-phase tripping device 10 is formed of a combination of a heater-mounted main bimetal 12 connected to each phase of a main circuit, a differential shifter mechanism 13 linked with an operating end of the main bimetal of each phase to detect operational displacement of the bimetal, and a temperature-compensating bimetal 14 interposed between the differential shifter mechanism 13 and a latch receiver incorporated in the opening-and-closing mechanism section 9 to transmit an output signal from the differential shifter mechanism to the latch receiver in order to trip the opening-and-closing mechanism section 9, with the temperature-compensating bimetal also acting as a tripping lever.
Furthermore, the differential shifter mechanism 13 includes a combination of a slide push shifter 15 and a pull shifter 16 located on opposite sides of the arrangement of the main bimetal 12 of each phase, and fitted and supported in a groove in an interphase partition wall 1b of the case 1, with an output lever 17 extending across the top surfaces of the push shifter 15 and pull shifter 16 and coupled to these shifters 15 and 16 with pins so as to move therewith. Arm portions projecting from the push shifter 15 and pull shifter 16 face each other, with the main bimetal 12 of each phase located between the arm portions.
The temperature-compensating bimetal 14 in the illustrated example is formed by bending a piece of bimetal like a hair pin, and has one end journaled on a bearing 18 linked with the adjustment dial 5 and the other end facing the output lever 17 of the above-described differential shifter mechanism 13. Moreover, an operating piece extending from near the bearing toward the opening-and-closing mechanism section 9 faces the latch receiver of the opening-and-closing mechanism section.
When the overload tripping device configured as described above is operated, an overload current flows through the main circuit to bend the main bimetal 12 in a predetermined direction, and the push shifter 15 and pull shifter 16 of the differential shifter mechanism 13 are displaced so as to follow the bending of the main bimetal 12. Then, the output lever 17 pushes a tip of the temperature-compensating bimetal 14. Therefore, the temperature5 compensating bimetal 14 rotates clockwise around a shaft support point of the bearing 18, causing the operating piece thereof to push the latch receiver to a released position. Synchronously with this movement, the opening-and-closing mechanism section 9 is tripped to open the movable contact shoe 8a of the breaking section 8, thereby interrupting the main circuit current. Furthermore, if an open phase occurs in the main circuit, the push shifter 15 and pull shifter 16 of the differential shifter mechanism 13 differentially operate to cause the output lever 17 to rotate counterclockwise around the pin coupling the output lever to the pull shifter. Thus, the temperature-compensating bimetal 14 is pushed to trip the circuit breaker, as described above.
Since the driving force or displacement of the main bimetal, which is effected when the main bimetal is bent to swing the temperature-compensating bimetal 14, is small, the latch receiver of the opening-and-closing mechanism section 9 is designed to move to its release position under a weak driving force.
In contrast, the electromagnetic instantaneous tripping device 11 includes a tripping coil 11a connected to the main circuit, a yoke 11b, a plunger 11c, and a tripping lever 11d following a movement of the plunger 11c. When an overcurrent flows through the main circuit due to a short circuit or the like, the plunger 11c carries out a sucking or pulling operation, causing the tripping lever lid to release the latch receiver of the opening-and-closing mechanism section 9, thereby instantaneously tripping the circuit breaker.
Furthermore, the temperature-compensating bimetal 14 in the illustrated example is bent like a hairpin and has one pivotally supported end. However, the temperature-compensating bimetal may be linearly formed so as to have a longitudinally intermediate point thereof journaled by the bearing so as to swing therearound, with one end thereof facing the output lever of the differential shifter mechanism and the other end facing the latch receiver of the opening-and-closing mechanism section via the intermediate lever. Alternatively, the tip of the temperature-compensating bimetal may extend toward the latch receiver without using the intermediate lever, or another metallic arm portion may be welded to the tip of the temperature-compensating bimetal so as to directly push the latch receiver, thereby allowing signals to be transmitted more efficiently.
In the above conventional configuration for the temperature-compensating bimetal, also acting as the tripping lever, opposite sides extending from the swinging shaft support point thereof toward the differential shifter and the latch receiver, respectively, have different lengths and masses.
In this case, if the temperature-compensating bimetal is linearly formed so as to have the longitudinally intermediate point journaled by the bearing to swing therearound, and the tip of the temperature-compensating bimetal is extended toward the latch receiver or another metallic arm portion is welded to the tip of the temperature-compensating bimetal so as to face the latch receiver, then the tip side with respect to the swinging shaft support point has a larger mass and thus a larger inertial moment than the opposite end. Consequently, the opposite sides of the temperature-compensating bimetal with respect to the swinging shaft support point are imbalanced.
Therefore, if an external stimulus, e.g., vibration or impact, is applied to a circuit breaker during use, the temperature-compensating bimetal may swing around the swinging shaft support point like a pendulum due to the difference in the inertial moment described above, pushing the latch receiver of the opening-and-closing mechanism section to its released position. Thus, although no overload current is flowing through the main circuit, the circuit breaker may be tripped to interrupt the main circuit.
The present invention is designed to resolve these issues, and an object of the invention is to provide an overload tripping device for a circuit breaker having an improved vibration-resisting performance so as to prevent the circuit breaker from being inadvertently tripped due to an external stimulus, such as a vibration or impact.
To attain this objective, the present invention provides an overload tripping device for a circuit breaker formed of a combination of a main bimetal for detecting overloads, a differential shifter mechanism linked with the main bimetal, and a temperature-compensating bimetal interposed between an output end of the differential shifter mechanism and an opening-and-closing mechanism section to transmit an output signal from the differential shifter to a latch receiver of the opening-and-closing mechanism section. The temperature-compensating bimetal also acts as a tripping lever, and has an longitudinally intermediate portion journaled so as to swing therearound and one end facing an output lever of the differential shifter mechanism. An operating arm made of a molding material, such as plastic, that has a lower density than metal is attached to a tip of the temperature-compensating bimetal, so that the tip of the temperature-compensating bimetal faces the latch receiver of the opening-and-closing mechanism section via the operating arm (first aspect of the invention).
With this configuration, the tip side of the temperature-compensating bimetal is lighter than the conventional structure, which has a portion pushing the latch receiver made of metal, thus reducing the difference in an inertial moment between the opposite sides of the bimetal with respect to the swinging shaft support point to maintain a substantially even balance between these sides. This, in turn, reduces the swinging vibration of the temperature-compensating bimetal induced by an external stimulus, thereby allowing the circuit breaker to resist vibration properly when it is inadvertently tripped.
Furthermore, according to the present invention, in order to further improve the vibration resistance of the overload tripping device, the temperature-compensating bimetal has a balance weight additionally installed thereon to correct imbalance between the inertial moments on opposite sides thereof with respect to the shaft support point thereof (second aspect of the invention).
By adding the balance weight to the temperature-compensating bimetal, the above-described balance of the inertial moment can be further enhanced. The balance weight is made of metal and is attached to an optimal position, depending on the degree of deviation of the center of gravity of the temperature-compensating bimetal that deviates from its correct position.